patch 8.0.0361: GUI initialisation is not sufficiently tested

Problem:    GUI initialisation is not sufficiently tested.
Solution:   Add the gui_init test. (Kazuki Kuriyama)
diff --git a/src/testdir/gui_init.vim b/src/testdir/gui_init.vim
new file mode 100644
index 0000000..bc5e33c
--- /dev/null
+++ b/src/testdir/gui_init.vim
@@ -0,0 +1,5 @@
+" gvimrc for test_gui_init.vim
+
+if has('gui_athena') || has('gui_motif') || has('gui_gtk2') || has('gui_gtk3')
+  set guiheadroom=0
+endif